The present invention relates generally to a measuring method and apparatus, and more particularly to a measuring method and apparatus for measuring an optical performance, such as a wave front aberration and the best focus position, of a projection optical system for transferring a pattern on a mask onto an object, and an exposure method and apparatus that are corrected using the measuring method and apparatus. The inventive measuring method and apparatus are suitable, for example, for measurements of the projection optical system used for a step-and-scan exposure apparatus or a scanner. The step-and-scan exposure apparatus exposes a mask pattern onto a wafer by continuously scanning the wafer relative to the mask, and by moving, after one shot of exposure, the wafer stepwise to the next exposure area to be shot.
A projection exposure apparatus has conventionally been used to transfer a pattern on a mask or a reticle onto an object to be exposed in manufacturing semiconductor devices in the photolithography process. A type of the exposure apparatus is shifted from the step-and-repeat exposure apparatus (“stepper”) to the scanner. The exposure apparatus is required to transfer a pattern on the reticle onto the object at a predetermined magnification with precision, and exposure at the best focus position using a projection optical system having a reduced aberration is important. In particular, due to the recent increasing demands for the finer processing to the semiconductor devices, the transfer pattern is sensitive to the aberration of the optical system. Therefore, there is a demand for highly precise measurements of the best focus position and wave front aberration of the projection optical system.
The instant inventor has already proposed one solution method for this problem for imaging a pattern on the reticle with the light irradiated by an illumination optical system, and for measuring a wave front aberration of the projection optical system based on a positional offset between a reference position and a center of the formed image. See, for example, Japanese Patent Application, Publication No. 2003-178968.
Other prior art references include Japanese Patent Applications, Publication Nos. 2003-318090 and 2002-289494, and U.S. Pat. Nos. 5,828,455 and 5,978,085.
Japanese Patent Application, Publication No. 2003-178968 maintains the mask stage and the wafer stage still during the measurement. The instant inventor has discovered that this method is suitable for the stepper but not always suitable for the scanner. There are two types of measurements for the scanner: One maintains these stages still during the measurement, and the other dynamically scans these stages during the measurement. The measurement of the best focus position at the scan state is different from that at the still state in that the measurement at the scan state needs to measure the control performance of the exposure apparatus in the focus direction (or the Z direction) in a real-time basis during scanning. In measuring the aberration of the projection optical system, the mask is scanned with a normal slit illumination (while the wafer is also scanned at the same time), and the entire exposure area on the mask is exposed. The slit has a predetermined width in the scan direction, and the resist image depends upon the integration of rays from an image point range corresponding to the slit width. Unlike the measurement at the still state, the aberration of the projection optical system in the measurement at the scan state should address an integration of the aberrations in the image point range corresponding to the slit width, rather than the aberration for one image point. Therefore, Japanese Patent Application, Publication No. 2003-178968 and other prior art references, which maintain the stages still during the measurement, cannot precisely measure the best focus position and the wave front aberration of the projection optical system with precision.